Field of the Disclosure
The present disclosure relates to an extensometer wherein the actuation of the arms is performed by a separate rotating bar.
Description of the Prior Art
An extensometer is an instrument that very accurately measures the extension of a test specimen in order to better capture the material properties of the specimen. A common extensometer is of the contacting type, which physically tracks two points in the specimen as the test takes place. These contacting points are typically in the form of arms that reach from the main body of the extensometer to the specimen location.
For automatic contacting extensometers, it is necessary to have a means to make the contacting arms attach to and detach from the test specimen. Existing products make use of actuators, usually electric motors that are mounted to each of the two moving arms.
During a test, the load cell on a test frame measures the force that is applied to the specimen. Since the arms are contacting the specimen, the load cell output will include not just the force applied to the specimen but also the additional force required to move the contacting arms. The force required to move these arms is called the running force. The running force also causes the contacting arms to deflect, causing errors in the extensometer readings. For these reasons, minimizing the running force is crucial to load output accuracy and extension accuracy.
The idea of an external bar being used to remotely actuate the arms has been used before in an extensometer, notably the MFL by Mess & Feinwerktechnik (MF) shown on FIG. 1. The device in FIG. 1 does not use a linear encoder to measure the displacement of the arms.
The actuation method employed in FIG. 1 is based on the use of two eccentrically-mounted shafts that operate in a cam-action as shown in FIG. 2. Driven from a common actuator shown in FIG. 3, the cam bars move synchronously until the high point rotates into contact with a tab on each arm of the arm pair, forcing the arms open as shown in FIG. 4.
Another prior art approach is to have a system where sensitive strain gauges are attached to the ends of the arms. The device then lets the strain gauges track the displacement of the specimen until the end of the very limited travel of the strain gauges is about to be reached, at which point the instrument uses actuators to move the arms in the direction of specimen elongation to provide slack and allow more strain gauge travel. As a result, this architecture is immune to increased friction. The arm location is driven by motors even during testing, and not by the specimen itself.